Endogenous hormone levels affect the regeneration ability of callus derived from different organs in barley

Hordeum vulgare (barley) is an important agricultural crop worldwide. A simple and efficient transformation system is needed to analyze the functions of barley genes and generate lines with improved agronomic traits. Currently, Golden Promise and Igri are the most amenable barley cultivars for stable transformation. Here we evaluated the regeneration ratios and endogenous hormone levels of calli derived from various malting barley cultivars, including Golden Promise, Haruna Nijo, and Morex. We harvested samples not only from immature embryos, but also from different explants of juvenile plants, cotyledons, coleoptiles, and roots. The callus properties differed among genotypes and explant types. Calli derived from the immature embryos of Golden Promise, which showed the highest ratio of regeneration of green shoots, had the highest contents of indoleacetic acid, trans-zeatin, and cis-zeatin. By contrast, calli derived from the cotyledons of Morex and the immature embryos of Haruna Nijo had elevated levels of salicylic acid and abscisic acid, respectively. We thus propose that the former phytohormones are positively associated with the regeneration ability of callus but the later phytohormones are negatively associated.